This invention relates to a device for non-destructive measurement of the extent to which a surface layer of a material has been affected by a superficial treatment, and particularly but not exclusively the measurement of the depth to which a material is affected by a thermal and/or theremochemical treatment.
The devices envisaged by the present invention are those which comprise a semi cylindrical block intended to be applied with its non-arcuate face against the sample to be studied, a transducer-emitter of ultrasonic waves, a transducer-receiver, the axes of the emitter and receiver being located in a common plane normal to the axis of the semi cylindrical block, and the emitter and receiver each being carried by a slide which is symmetrically movable on the arcuate surface of said block, means for entrainment of both transducers, and means for measuring the angle which they form between them.
This apparatus is based on a method which includes emitting a beam of ultrasonic waves and appropriately directing the beam so as to produce surface waves on the material to be studied.
Detection and measurement are effected on the beam reflected by the sample surface on which the control operates. This method enables determination on the value of the speed of propagation of the Rayleigh waves on the surface of a solid by measurement of the angle of incidence of the ultrasonic beam causing the production of these surface waves. This theory is particularly developed in the works of G. Bradfield, to which reference will be made for further information, particularly to the article entitled "The ultrasonic goniometer and its applications" in the February issue, 1968, of the periodical "Non-Destructive Testing" and to British Pat. Nos. 959.029 and 772.083.
In the present case, the principle involves taking into account the fact that the speed of propagation of these surface waves is affected by the presence of a layer obtained, particularly on steel, by a thermal treatment, and in particular this variation in speed depends on the value of the thickness of said layer.
A generator delivers the electrical signals necessary to supply the transducer-emitter. The ultrasonic waves may be generated either in the form of short-duration pulses or in wave trains of regulable length; in both cases, various frequencies may be used.
The measurement assembly enables observation and analysis of the signals received by the transducer-receiver. This assembly, made up principally of an oscilloscope, may be accompanied by a recording instrument. In known devices of the type in question, the mechanical connection ensuring symmetrical movements of both transducers comprises a fairly large number of toothed sprockets and pinions, and sliding cardan-jointed shafts, rendering the assembly relatively complicated and liable to have a degree of operational play the elimination of which would make the apparatus even more complex.
One of the objects of the invention is to obviate or mitigate these disadvantages by providing an apparatus of very simple construction, which is robust, convenient to use and reliable and precise in operation.